The present invention relates to an apparatus and a method for measuring, in several directions, deformations of core samples taken from geological formations, which deformations particularly being from the relaxation of the stresses which were exerted on these cores before being taken.
A method and apparatus of the aforementioned type is disclosed in commonly-assigned co-pending U.S. application Ser. No. 07/213,139, filed Jun. 29, 1988.
The present invention makes it possible to determine the tensors of stresses and/or deformations and, in particular, the direction of the minimum horizontal compression stress and, with the knowledge of this minimum horizontal compression stress, applied to mining, oil or gas production, particularly in low permeability formations, it is possible to define a plane of propagation of hydraulic fractures which are perpendicular thereto.
By determining this stress from several core samples taken at different places in a geological formation, it is possible to determine the optimum position of the well or boreholes for improving the production of a field under development.
By permeability is meant the property of the inner spaces of a rock to communicate together, which makes it possible for the fluids which they contain to move through the rock in question.
One way of knowing the condition of the stresses and/or deformations and/or more particularly this plane of propagation consists, in taking a rock core sample from the geological formation concerned, in measuring from this core sample the deformations resulting from the substantially instantaneous relaxation of the stresses in situ.
Once the sample has been taken from the formation, the formation first of all undergoes instantaneous and considerable deformations which are only accessible to measurement if this measurement is made at the bottom of the well just at the time the sample is taken.
These instantaneous deformations are succeeded, for several tens of hours, by delayed deformations whose order of size is smaller than that of the instantaneous deformations, and which may, for example, be observed once the core sample has been raised to the surface of the ground, as may be the case in oil drilling.
One method, called relaxation, consists in measuring the delayed deformations of a core sample in several directions and deriving therefrom the set of deformations and their evolution time and, consequently, to determine the stresses to which the formation considered is subjected.
If, for the sake of facility, these measurements are made on cylindrical core samples and if the axis of the core sample is considered as one of the main deformation directions (a simplifying assumption, justified when the core samples are taken from vertical wells or taken perpendicularly to the stratification), four independent directions (one parallel to the axis of the sample and three in a plane perpendicular to this axis) are sufficient for determining with a rheological model, such as those presented by Blanton and Warpinsky, the deformations of a core sample, their evolution in time and the stresses of the formation studied.
However, because particularly of the very low value of the deformations measured (a few micrometres) and because very often of the operating conditions for obtaining these latter (vibrations of the site, wind, . . . producing parasite movements), and also because of the device for acquiring these measurements, the measurements of the deformations, like the deductions made therefrom, are erroneous.
If the parasite movements of the sample adversely affect the correct measurement of the deformations of the sample, other parasite phenomena may completely mask the deformations induced by relaxation of the stresses. These phenomena are more particularly the temperature variations, even very small, of the core sample and/or of the measuring assembly, the fluid saturation variations of the sample.